The question of which animal holds the title of the smallest in the world is more complex than it first appears. The term “smallest” can refer to various measurements, such as length, weight, or even volume, and the record holder often depends on the specific criterion used. This highlights the diversity of life and adaptations allowing organisms to thrive at extreme sizes.
Defining Smallness
Defining an animal as the “smallest” requires careful consideration of the measurement being applied. Body length, typically measured from snout to tail or snout to vent, is a common metric, but it can vary significantly between sexes or life stages within a single species. Mass or weight offers another perspective, often revealing different record holders, as a short but dense animal might outweigh a longer, more slender one. Volume is yet another, less frequently used, criterion.
Challenges in consistent measurement arise from the delicate nature of tiny specimens, the difficulty of finding them, and the variations in how different researchers define “length” or “weight” for microscopic organisms. Thus, multiple animals can legitimately claim the title of “smallest” depending on the context.
The Smallest Vertebrates
The vertebrate subphylum, characterized by a backbone, includes some remarkably tiny creatures across its diverse classes.
Mammals
Among mammals, the Etruscan shrew (Suncus etruscus) is considered the smallest by mass, weighing around 1.8 grams on average, though individuals can range from 1.3 to 2.5 grams. Its body length typically measures between 3 to 5.2 centimeters, excluding its tail. In contrast, the bumblebee bat (Craseonycteris thonglongyai) often holds the record for the smallest mammal by length, typically measuring 29 to 33 millimeters and weighing between 1.7 to 2 grams.
Birds
The bee hummingbird (Mellisuga helenae), native to Cuba, is the smallest bird, with males measuring about 5 to 5.7 centimeters in length and weighing as little as 1.6 to 1.95 grams. Females are slightly larger, reaching about 6.1 centimeters and 2.6 grams. This species is often mistaken for a bumblebee due to its minuscule size.
Reptiles
For reptiles, the chameleon Brookesia nana, also known as the nano-chameleon, is currently recognized as the smallest. A male specimen measured just 13.5 millimeters from snout to vent, with a total length of 21.9 millimeters including its tail. Females of this species are slightly larger.
Amphibians
Among amphibians, the frog Paedophryne amauensis from Papua New Guinea is considered the smallest, measuring a mere 7.7 millimeters in snout-to-vent length. This tiny frog weighs approximately 0.01 grams.
Fish
In the aquatic realm, several fish species contend for the title of smallest. The stout infantfish (Schindleria brevipinguis) is a strong contender, with males reaching maturity at 6.5 to 7 millimeters and females at 7 to 8 millimeters. The largest known specimen is only 8.4 millimeters long and weighs less than 1 milligram. Another notable contender is Paedocypris progenetica, a carp-relative from Indonesian swamps, where mature females measure around 7.9 millimeters in length.
The Smallest Invertebrates
Invertebrates encompass a vast array of life forms, and many of the smallest animals belong to this group, often pushing the boundaries of microscopic existence.
Insects
Among insects, fairyflies (family Mymaridae) are exceptionally tiny. The male Dicopomorpha echmepterygis is recognized as the smallest known insect, measuring only 0.139 millimeters in length. These males are wingless and eyeless. The smallest known flying insect is also a fairyfly, Kikiki huna from Hawaii, which is just 0.15 millimeters long.
Crustaceans
The crustacean Stygotantulus stocki holds the record for the smallest crustacean and potentially the smallest arthropod. This ectoparasite measures less than 0.1 millimeters, specifically around 0.094 millimeters in length.
Arachnids
For arachnids, the spider Patu digua from Colombia is considered the smallest. Adult females measure approximately 0.37 millimeters in body length, while the males are even smaller, reaching a minuscule 0.25 millimeters. Another contender, Patu marplesi from Samoa, has male legspans as small as 0.46 millimeters.
Other Microscopic Invertebrates
Beyond these groups, some of the most minute multicellular animals are found among the myxozoans. These highly specialized cnidarians, related to jellyfish, are obligate endoparasites. Some myxozoan species, such as Myxobolus shekel, measure a mere 8.5 micrometers (0.0085 millimeters) when fully grown. Rotifers, also known as wheel animalcules, are another group of microscopic multicellular animals, with some freshwater species not exceeding 50 micrometers (0.05 millimeters) when fully grown.
Life at the Limits: Challenges of Being Tiny
Existing at such diminutive scales presents unique biological and physical challenges. A significant factor is the surface area to volume ratio, which increases dramatically as an organism gets smaller. This high ratio means tiny animals lose heat and water more rapidly than larger ones. To compensate, they often require high metabolic rates, consuming proportionally more food to maintain energy.
Fitting complex organ systems into a microscopic body poses challenges, especially for the nervous system and sensory organs. Miniaturization can lead to simplified body plans or specialized adaptations. The large surface area also makes tiny animals susceptible to desiccation, requiring them to inhabit humid microclimates. At their scale, physical forces like air resistance and surface tension become more significant, influencing movement and interaction with their environment.